The present invention relates to a shift arrangement for motor vehicle transmissions and, more particularly, to such an arrangement for reducing operator shift effort when shifting from a two-wheel drive mode to a four-wheel drive mode under cold weather conditions.
In general, power transfer mechanisms are used in association with both manual and automatic transmissions for selectively directing power to the wheels of a motor vehicle upon shifting from the two-wheel drive mode to the four-wheel drive mode. It is known to use a gear synchronizer clutch arrangement in part-time transfer cases to provide "shift-on-the-fly" two-wheel drive to four-wheel drive "mode" shifting. One example of such an arrangement is shown and described in U.S. Pat. No. 4,770,280 issued Sept. 13, 1988 to Frost which discloses a part-time transfer case having a gear synchronizer clutch unit incorporated as part of its "mode" shifting mechanism. Following speed synchronization, a fork mechanism shifts the synchronizer clutch sleeve into engagement with the external splines of a silent chain carrier for locking the transfer case in its four-wheel drive mode thereby delivering power via the drive chain to the vehicle's front output shaft.
In part-time four-wheel drive systems, various axle disconnect systems have been used for enabling the non-driven wheel(s) to rotate free of the remainder of its associated axle final drive assembly when the vehicle is operating in the two-wheel drive mode. Conventionally, "shift-on-the-fly" part-time transfer cases utilize automatic locking hubs having manual, electrical or fluid (i.e. vacuum) actuated front axle disconnect systems. These "automatic" systems selectively connect the front wheels to its associated front drive mechanism for transferring drive torque from the transfer case to the front wheels. Typically, these systems are actuated to couple the front drive components in response to a signal indicating that the vehicle operator has shifted into the four-wheel drive mode.
Prior U.S. patents disclose various means for enabling the non-driven wheel(s) to rotate free of the remainder of the front drive mechanism when in two-wheel drive mode. For example, U.S. Pat. No. 2,913,929 (Anderson) discloses a front axle design having complementary mechanism for cooperatively disconnecting both of the front wheels from the drive train to permit two-wheel drive operation. These complimentary mechanisms are actuated in common by a movable lever in the passenger compartment of the vehicle. The movable lever is connected to complementary clutch collars which are shiftable between two-wheel drive and four-wheel drive positions on the right and left axle shafts. Likewise, U.S. Pat. No. 2,770,150 (Culverwell) discloses a front axle design for a four-wheel drive vehicle which includes a mechanism for simultaneously disconnecting the right and left axle shafts from the center differential. In this type of system, disconnection of the front wheels takes place by shifting complementary clutch collars located within the front differential housing. Finally, reference may be had to U.S. Pat. No. 4,381,828 issued May 3, 1983 (Lunn et al) for details of a vacuum actuated axle disconnect system.
In conventional part-time four wheel drive systems, once synchronization of the front and rear drive assemblies is completed, the synchronizer clutch sleeve is moved into engagement with the chain carrier external splines. Upon complete "lock-up" therebetween, a vacuum switch signals the axle disconnect system to move a clutch collar for locking the front wheels to the front drive mechanism.
A common problem associated with part-time four-wheel drive transfer cases is that, during cold weather conditions, the vehicle operator must physically overcome excessive shift resistance and noise, commonly referred to as "clash", when attempting to shift into the four-wheel drive mode. The increased shift effort is due primarily to the high viscosity of the cold axle lubricant entrained within the front drive assembly and the inertial loading of the front prop shaft which must be overcome by the "mode" shift synchronizer clutch unit. More specifically, the cold lubricant detrimentally effects the capability of the synchronizer clutch unit to develop adequate "index" torque required to overcome the front axle drag torque. As such, the front output components of the transfer case slow down following synchronization to produce relative movement between the clutch sleeve internal splines and the chain carrier sprocket splines. This relative movement causes the splines to "clash" causing the undesirable noise and shift effort.